This application claims the benefit of Korean Application No. 99-10272, filed Mar. 25, 1999, in the Korean Patent Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a substrate of an optical disc with lands and grooves, and more particularly, to an optical disc substrate with deep grooves having a depth of xcex/4n to xcex2n, where xcex is the wavelength of a laser beam emitted from an optical pickup to record/read index from the optical disc and n is the refractive index of the optical disc substrate.
2. Description of the Related Art
Optical discs are information recording media adopted by a disc player, which writes and/or reads information in a non-contact manner. The need for a high recording density at a limited data recording region has provided a suggestion for an optical disc substrate which allows data writing on both its grooves and lands.
FIG. 1 is a schematic view of an existing optical disc substrate adopting a land-and-groove recording method. As shown in FIG. 1, an optical disc substrate 1 comprises a plurality of tracks spirally formed from the center to the periphery of an optical disc, alternately forming a plurality of grooves 3 having a predetermined depth and a plurality of lands 5 having the same level as the surface of the optical disc substrate.
In particular, the format book for a 2.6-gigabyte DVD-RAM suggests a ratio of the land width and the groove width be approximately 50:50. A land-and-groove recording method applied to such an optical disc having the above configuration is advantageous in that a difference in height between lands and grooves reduces crosstalk, which is noise generated from adjacent tracks, and writing on both lands and grooves increases the recording density. Another advantage of the land-and-groove recording method is a larger amplitude of a pushpull signal compared to a recording method which allows writing on only either lands or grooves. This larger amplitude of the push-pull signal is because an optical track pitch which causes the push-pull signal is half as small as a data track pitch.
For a high recording density in optical discs having the above configuration, the track pitch (TP) must be reduced. In this case, the size of a write beam spot must be reduced to keep writing and reading characteristics. However, as the recording capability of optical DVDs increases, a relative track pitch with respect to the write beam spot size decreases, which is shown in Table 1, causing xe2x80x9ccross erasexe2x80x9d which refers to erasure of signals on adjacent tracks, and thus limiting the increase in recording density.
The cause of cross erase can be summarized into two factors. One is thermal absorption of the write beam by adjacent tracks, and the other is thermal transfer to adjacent tracks during writing. The thermal transfer between recording layers in an optical disc, which causes a temperature increase in the optical disc can be avoided by spacing adjacent tracks further apart.
In this way, an optical disc with deep grooves has been proposed. In a case such as an optical disc, the groove depth is larger than the groove depth Gd, xcex/6n, where xcex is the wavelength of a laser beam of an optical pickup and n is the refractive index of an optical disc substrate, of a general optical disc, which elongates the thermal conductive distance and in turn suppresses the occurrence of both cross erase and crosstalk. However, the problem with deep-groove optical discs is the phase reversion of a tracking error signal, the so called xe2x80x9cpush-pull signal,xe2x80x9d at a depth below that of a predetermined depth.
FIG. 2 shows the push-pull ratio (PPR), divided push-pull ratio (DPP) and on-track ratio (OTR) for a 4.7-gigabyte DVD-RAM with respect to the depth of grooves.
Phase reversal of the push-pull signal means that the deep grooves are tracked according to the tracking conditions for lands of a general optical disc as shown in FIG. 1. Since the tracking conditions for lands and grooves cannot be the same, the DVD-RAM format described with reference to FIG. 1 delimits the depth of grooves to be less than or equal to xcex/4n, where xcex is the wavelength of a laser beam for an optical pickup to record/read data on/from the optical disc and n is the refractive index of an optical disk substrate, which permits the same phase of push-pull signals for lands and grooves.
The crosstalk signal and push-pull signal are influenced by a slant angle xcex8 of the grooves. As shown in FIG. 1, the slant angle xcex8 of grooves refers to the angle between a top surface of the lands and the groove sidewalls (or also described as a lateral extension of the lands or the grooves and an extension of the groove sidewalls).
FIG. 3 illustrates the crosstalk and the push-pull signal for an optical pickup adopting an objective lens having a numerical aperture (NA) of 0.6 with respect to the groove depth when the slant angle xcex8 of grooves is 60xc2x0 and 80xc2x0. In FIG. 3, A and B indicate the crosstalk signals at the slant angle xcex8 of grooves of 60xc2x0 and 80xc2x0, respectively. C and D indicate the push-pull signals at the slant angle xcex8 of grooves of 60xc2x0 and 80xc2x0, respectively.
For the result of FIG. 3, the groove depth has been normalized based on the wavelength (xcex) of the incident laser beam and the refractive index (n) of the disc substrate. In FIG. 3, the horizontal dashed line at the push-pull signal of 0 indicates an optical groove depth of xcex/4n. Thus, it can be concluded that the optical groove depth for a predetermined wavelength of a laser beam varies depending on the slant angle xcex8 of grooves even at the same physical groove depth (the physical groove depth is an absolute depth without taking into account the refractive index n and the wavelength xcex of the optical disc substrate, and the optical groove depth is the depth taking n and xcex into account). Also, as previously mentioned, the phase of the push-pull signal reverses around the optical groove depth of xcex/4n. Also, the push-pull signal of deep grooves varies depending on the slant angle xcex8 of grooves.
FIG. 3 shows that when the physical depth of the grooves is small, the effect of the slant angle xcex8 of the grooves on the variation of the push-pull signal and the crosstalk signal is negligible, compared to that of the groove depth. However, as the physical depth of grooves increases, variations of the push-pull signal and crosstalk signal with respect to the slant angle xcex8 of grooves, i.e., at 60xc2x0 and 80xc2x0, increase.
To solve the above problems, it is an object of the present invention to provide an optical disc substrate with deep grooves having a depth of xcex/4n or more, where xcex is the wavelength of a laser beam from an optical pickup and n is the refractive index of a substrate, which results in improved crosstalk and cross erase characteristics.
Additional objects and advantages of the invention will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention.
The objects of the present invention is achieved by an optical disc substrate for land-and-groove recording, comprising: a plurality of deep grooves having a predetermined depth, individual ones of the deep grooves having sidewalls slanted at an angle of xcex8; and a plurality of lands having the same level as the surface of the optical disc substrate, wherein a depth D of each of the grooves for minimum crosstalk is determined by the following mathematical relation [1]
D=0.4022xe2x88x920.4574xc3x97A+0.6458xc3x97A2xe2x80x83xe2x80x83[1]
where
D=physical groove depthxc3x97{fraction (n/xcex)}      A    =                            NA          ·          TP                λ            xc3x97              1                              sin            2                    ⁢          θ                      ,
TP indicates the track pitch of the optical disc substrate,
NA indicates a numerical aperture of an objective lens of an optical pickup,
xcex8 is the slant angle of the grooves between a top surface of lands and sidewalls of the grooves,
xcex is the wavelength of a laser beam of the optical pickup which is radiated on the optical disc through the objective lens for the land-and-groove recording, and
n is the refractive index of the optical disc substrate.